The present invention relates to digital communication and more particularly to synchronization of certain parameters between a receiver and a transmitter.
In an OFDM (orthogonal frequency division multiplexing) communication system, data is communicated in a series of time domain bursts. To form each time domain burst, an IFFT is applied to a group of frequency domain symbols and a cyclic prefix is added to the transform result prior to transmission. Transmission may involve conversion to an analog signal, conversion to an intermediate frequency (IF), then upconversion to a desired selectable carrier frequency prior to final amplification and propagation across a transmission medium. Upconversion is typically achieved by mixing the IF signal with a variable frequency oscillator signal. The carrier frequency is varied by varying the oscillator frequency.
On the receiver end, preamplification is followed by downconversion to IF from the carrier frequency, again by mixing with a variable frequency oscillator. The resulting IF signal is typically converted to a baseband digital symbol sequence. The cyclic prefix is removed and an FFT is applied to recover the original frequency domain symbols.
For successful communications, certain parameters must be synchronized between the transmitter and the receiver. For example, the transmitter and receiver must have a common and precise shared understanding of the transmission frequency. In the exemplary system described above, this means that the variable frequency oscillators at the transmitter and receiver should be locked to each other. Imprecision with respect to the transmission frequency will cause inaccurate recovery of the OFDM symbols. To maintain system performance, it is desirable to always maintain frequency offset between the transmitter and receiver to within 1% of the spectral width occupied by a single frequency domain OFDM symbol.
When the receiver initially acquires the transmitter, it is desirable that the synchronization system tolerates and corrects a very wide misalignment between the transmitter and receiver oscillators. This allows the use of much lower cost analog components.
Also since OFDM communication proceeds on a burst by burst basis, the receiver and transmitter must agree on exactly when each burst begins. Again, the consequence of missychronization will be lost data.
Additionally, the length of the cyclic prefix used by the transmitter will depend on the transmitter's understanding of the duration of the transmission channel's impulse response. This information may not necessarily be immediately available to the receiver in systems where the transmitter is a hub of a point to multipoint system. In these cases, the modulation parameters, including the cyclic prefix, may be programmable at the hub and each subscriber unit must “turn on” and acquire this OFDM modulation parameter. Upon acquisition, the receiver therefore must also determine the cyclic prefix length used by the transmitter.